The present invention relates to a method of making thick-and-thin fibers wherein the thick-and-thin ratio is sufficiently high, the thick-and-thin recurring length is short and uniform, and the phases of the thick-and-thin profile among multi-filaments are practically identical.
Thick-and-thin fibers whose fineness varies along the longitudinal direction have been known. They are used, for example, in pile fabrics so as to simulate natural furs because the profile and the fiber properties of the thick-and-thin fibers are similar to the hair of natural furs.
Thick-and-thin fibers conventionally made of synthetic polymers, however, have so small a thick-and-thin ratio that is, of less than four, that they are insufficient to be used to simulate the hair, for example, of natural mink fur. Since it was difficult to produce the thick-and-thin multi-filaments or tow with a uniform length between the adjacent thick portions and a good phase coherency of the thick-and-thin profile among filaments, only dissimilar thick-and-thin staple fibers resulted when the thick-and-thin filaments or tow were cut to a constant fiber length. This was one of the serious drawbacks against producing good simulated furs with excellent appearance and hand.
The Japanese Patent Publication No. 52-47053, for example, describes a process for the preparation of thick-and-thin fibers wherein undrawn filaments are heated intermittently, and drawn at a constant speed to a predetermined length to stretch selectively the heated region with the result that a tapered thin portion is formed at the heated region. The filaments are further drawn and cut into the thick-and-thin staple fibers which are characterized by the following formulae: EQU l/D&gt;30, and d=d'&lt;0.5D,
wherein
D is the fiber diameter at the thick portion, PA1 d is the fiber diameter at the thin portion between the adjacent thick portions, PA1 d' is the fiber diameter at a sharpened end, and PA1 l is the length from the middle thin portion up to the sharpened end.
This process may be useful to render a monofilament into a thick-and-thin fiber with a relatively uniform thick-and-thin recurring length, but it seems to have difficulty in manipulating multi-filaments or tow because it is quite difficult to heat each filament at the same corresponding position. Therefore, good phase coherency of the thick-and-thin profile cannot be attained. In addition, since it becomes more difficult to draw the heated portion selectively as the fiber fineness increases, thick-and-thin fibers whose thick-and-thin ratio is more than four becomes technically difficult.
A method of producing a filament having uniformly recurring symmetrically tapered portions has been disclosed in U.S. Pat. No. 2,418,492, which comprises extruding an organic filament forming material in a molten condition at a constant volume rate through a spinneret across an air gap into a liquid cooling bath maintained at a temperature below the temperature of solidification of said material, and directly withdrawing the filament formed in accordance with a repetitive schedule of linear rates comprising a period of acceleration, a period of deceleration of greater duration than that of acceleration, and a period of uniform withdrawal. This method may enable us to produce thick-and-thin fibers with a good phase coherency of the thick-and-thin profile among filaments, but it is impossible to combine the extrusion process with the drawing process which is necessary for good end use because the non-uniform withdrawal, which is intentively applied, persists all over the running filaments in the process.
It is well known that there are many methods to vary the fiber fineness along its longitudinal direction, including a periodical change in the throughput, take-up velocity or spinning length during the spinning process; or a periodical change in drawing ratio or length of the drawing zone during the drawing process; or an intermittent drawing to impart undrawn portions. Although these methods seem to be possible in principle, they betray their incapability when it comes to making the thick-and-thin fibers with the high thick-and-thin ratio herein intended, and they cannot be used from the entrepreneur's point of view because of the difficulties with machinery.
The periodical change in the throughput can be, for example, attained by the use of the so-called pulsating pump or the method described in the Japanese Patent Publication No. 43-20246, wherein the constant metering has been coupled with a periodical change in the resident volume before the extrusion. This method cannot be used to produce the thick-and-thin fibers with high thick-and-thin ratio and short recurring length as intended in the present invention because too intricate a machinery is required and also because the viscoelastic properties of the polymer concerned damps the effect of the change in the resident volume.
Another method of varying the take-up velocity requires intricate machinery for the periodical change in the peripheral velocity of the rotating rollers. Some devices for this end can be found, for example, in the U.S. Pat. No. 2,418,492 and British Pat. No. 1,1086,511. However, the change in the take-up velocity, as already pointed out, makes it difficult to combine the extrusion process with the drawing process which is necessary for good end use. The separation of the processes, therefore, becomes unavoidable, which results in a deleterious effect on the phase coherency of the thick-and-thin profile among multi-filaments. To obtain the thick-and-thin fibers herein intended, the take-up velocity should be periodically altered from about zero to several tens or hundreds of meters per minute, so that the rotating rollers have to swiftly change their rotating speed against their inertial force. This requirement may be temporarily realized in an experimental scale, but its permanent realization is so difficult that the industrial application of this method becomes impossible.
Another method of varying the spinning length in a melt spinning process consists of some manipulations by which the spinning length between the spinneret and the take-up device is altered. However, the spinning length in melt spinning is usually so great that the thick-and-thin fibers with high thick-and-thin ratio and short thick-and-thin recurring length as intended in the present invention cannot be attained.
Another method adopted in the drawing process is an imperfect drawing, wherein uniform undrawn filaments are drawn intentionally under a low drawing ratio so that some undrawn portions may be included. However, since there is a certain natural drawing ratio in which the imperfect drawing is realized, the thick-and-thin ratio is determined a priori and must remain at a low level. This is a fatal drawback for the present purpose. On top of that, this method, wherein undrawn portions of filaments are intentionally included by the application of a certain low drawing ratio less than the natural drawing ratio, is very poor at giving thick-and-thin fibers with a uniform thick-and-thin recurring length which is one of the important objects of the present invention. The vulnerability at the undrawn portions to a relatively small force or a chemical attack may also cause another problem in later processing.
As is apparent in the above explanation, the conventional miscellaneous methods cannot afford the thick-and-thin fibers intended in the present invention, wherein the thick-and-thin ratio is sufficiently high, the thick-and-thin recurring length is short and uniform, and the phase coherency of the thick-and-thin profile among filaments is also good.